Undersea slip menace for Kaikoura

The Cook Strait Canyon isn't the only underwater chasm that scientists worry could pose a serious threat to the country.

This year, Niwa ocean geologist Dr Joshu Mountjoy and colleagues explored the Kaikoura Canyon, which comes up to within 500m of the coast south of Kaikoura and feeds sediment into the 1500km-long Hikurangi Channel that runs east of New Zealand.

Modelling by Niwa scientists in 2006 considered the possible severity of a tsunami off the Kaikoura coast, triggered either by a submarine earthquake or landslide.

Landslides at the head of the canyon were thought to happen every 200 years on average, and modelling had shown a large one could result in a rapid-fire tsunami bearing down on South Bay with a crest 13m above sea level.

Dr Mountjoy returned to the head of the canyon with new technology this year to take another look at the size, shape and physical character of the sediment deposit.

His team spent three days at the site mapping the seabed and imaging and sampling it using equipment including a hull-mounted echosounder, and a multichannel boomer seismic reflection system that could probe 50m into sea-floor sediment.

He told Niwa's Water and Atmosphere publication that this enabled them to unravel 20,000 years of sedimentary deposits in the area.

The survey also used two coring techniques to collect seven sediment cores up to 1m long.

"We are analysing these to determine the rate at which sediment is accumulating on the seafloor," he said.

"The project team is now assessing the data collected over the three days to learn more about the way the sediment moves and accumulates in the canyon."